Handle requests when in not_connected to wait for retrying

[essen]

Against the current code I'm thinking this should be enough:

diff --git a/src/gun.erl b/src/gun.erl
index a548a5d..00c7cfd 100644
--- a/src/gun.erl
+++ b/src/gun.erl
@@ -1061,6 +1062,9 @@ handle_common(cast, Any, _, #state{owner=Owner}) when element(2, Any) =/= Owner
    element(2, Any) ! {gun_error, self(), {notowner,
        "Operations are restricted to the owner of the connection."}},
    keep_state_and_data;
+%% We postpone all HTTP/Websocket operations until we are connected.
+handle_common(cast, _, StateName, _) when StateName =/= connected ->
+   {keep_state_and_data, postpone};
 handle_common(Type, Event, StateName, StateData) ->
    error_logger:error_msg("Unexpected event in state ~p of type ~p:~n~w~n~p~n",
        [StateName, Type, Event, StateData]),

Thoughts?

[tony612]

@essen It's OK for me.

[tony612]

@essen I added a test and changed it same as your changes. Feel free to use your own code if you already finish it.

[essen]

I'm using your commit but rewrote the test to confirm the request actually goes through when the connection finally succeeds. Merged, thanks!